1. Introduction
As natural resources and assets, ecosystems play an indispensable role in the development and survival of human beings [
1]. Ecosystem services refer to the supply of various products and services required to meet the requirements of and maintain human life in natural ecosystems and ecological processes, which are closely related to human well-being [
2,
3]. The United Nations’ Millennium Ecosystem Assessment Plan divides the field into four categories: supply, regulation, culture and support services [
4]. Long-term acceleration of human resources development and land use to meet the growing population demand, exceeding the carrying capacity of the ecosystem, leads to ecological degradation and environmental problems such as habitat fragmentation, soil erosion and water pollution [
5,
6]. At present, the global action to prevent ecosystem degradation is emerging, which has stimulated a lot of exploratory research, and the study of ecological service function assessment and trade-off synergistic relationships has also become the research focus. It is vital to clarify the interactions among ecosystem services to improve regional coordination and sustainable development and achieve a “win—win” outcome with respect to ecological conservation [
7,
8].
At a certain scale, ecosystem services are not completely unrelated, but affect each other in a variety of complicated ways [
9]. The pros and cons of trade-offs and mutually reinforcing synergies are typical manifestations of this effect [
10,
11]. When an increase or decrease in one ecosystem service is accompanied by a decrease or increase in another ecosystem service, the relationship is referred to as a trade-off. When two ecosystem services increase or decrease simultaneously, it is referred to as synergistic [
12]. In the presence of such complex relationships, the changes of ecosystem services are closely related. The drastic changes of land use over the past 50 years have influenced the ecosystem pattern and prompted changes in ecosystem services and their relationships. Studying the spatial and temporal changes and interrelationships of ecosystem services and exploring their regional differences and level effects can provide a basic reference for regional land use decision-making and ecosystem service management [
13].
The present studies mainly used the theories of geography and ecology to perform qualitative analyses of ecosystem services, but quantitative studies on the service benefits were few [
14]. Researchers typically used statistical descriptions [
15], simulation of scenarios [
16] and quantitative modelling [
17] to conduct assessments at different spatial scales such as regions, watersheds, mountainous areas and ecological function areas, or different systems such as forests, farmland and grassland, then further analyzed the coupling relationship between the different functions. Marques et al. [
18] used the water yield (
WY) of the InVEST model to perform a functional assessment of the Francoli Basin in Spain and further explored the response of water production to climate change. Goldman et al. [
19] used the water yield and nutrient transport module in the InVEST model to evaluate the ecosystem service function in Colombia and analyzed the correlations in order to formulate a more scientific investment plan. Li et al. [
20] used the modules of water yield, soil conservation (
SC) and water quality purification in the InVEST model to study the impact of land use change on ecosystem services in the Miyun Reservoir Basin. Li et al. [
21] explored the changes of land use and ecosystem services in the mainstream and tributaries of the Weihe River Basin for assessment and classification with the help of the InVEST model; they also used a correlation analysis to analyze the trade-off and synergistic relationships between the various service functions. Yang et al. [
22] evaluated five ecosystem services, namely water yield, carbon storage (
CS), soil conservation, NPP and habitat quality, in the Yellow River Basin via the InVEST and CASA models and analyzed the trade-off for the relationships among the various ecosystem services. Most of the above scholars’ research on the ecosystem service assessment and the trade-offs and synergies were based on quantitative analysis of large and medium scale watersheds or administrative boundaries, while research tended to lack in the trade-offs and synergies of ecosystem services at the scale of small basins and multi-levels service functions. In addition, there was little research on the dynamic trends and driving mechanisms of the service changes (natural and anthropogenic inputs and impacts) under long-term sequences. Therefore, research on the trade-offs and synergies of ecosystem services needs to eliminate the previous boundary selection and single-scale model defects and carry out multi-scale and multi-type service correlation research to clarify the trade-offs and synergies and spatial differences among the multi-type services, so as to facilitate the effective implementation of ecosystem management.
The Yellow River Basin is an important ecological barrier area and economic zone in China, performing vital ecological functions and occupying an indispensable position in maintaining environmental security and social and economic development [
23]. The “Ecological protection and high-quality development of the Yellow River Basin” in the “14th Five-Year Plan” has been identified as a major regional development strategy in China, and strategic research results have emerged constantly. Additionally, progress has been made in various fields such as ecological protection [
24,
25], the industrial economy [
26,
27] and ecological efficiency [
28,
29]. The ecosystems of the upper, middle and lower reaches of the Yellow River Basin largely differ. The Yellow River Basin (Henan section) covers both the middle and lower reaches, among which soil erosion in the middle reaches is serious and the ecological flow in the lower reaches is lower [
30]. As a small watershed, the Yellow River Basin (Henan section) provides multiple types of ecosystem services and is a targeted study area. Most studies have been based on the whole basin or administrative boundaries, but targeted and in-depth research on the ecological functions and relationships of small basins at the spatial level is lacking.
In summary, previous studies on ecosystem services have focused on arid/semi-arid areas, mostly at large and medium scales such as the Yellow River Basin and the Weihe River Basin. There were few studies on the trade-offs and synergies between services in different land use types and the sequence was short. Therefore, this article eliminated previous studies based on the Yellow River Basin or administrative boundary, and selected key regions in the Yellow River Basin based on the watershed boundary to carry out the trade-off and synergy of long-term (1990–2020) and multi-scale (watershed and landscape) ecosystem services research for the semi-humid temperate climate zones, which provides a scientific and practical reference for the study of ecosystem services and the protection of mountains, forests, fields, lakes, grasses and sand in the semi-humid climate zones.
5. Conclusions
There were significant spatial and temporal diversities in land use and ecosystem services in the Yellow River Basin (Henan section) from 1990 to 2020. Our research revealed the following results: (1) The aggregation of land use structure in the Yellow River basin (Henan section) was significant. Over the past 30 years, the transfer of farmland in the basin has been dominant, with the largest rate of transfer being farmland area while the smallest was unused land. Among them, construction land increased with time, while the other categories were mainly outflow to other land use types. The conversions occurred mainly in grassland, woodland, farmland and construction land. (2) From 1990 to 2020, the water yields in the Yellow River Basin (Henan section) were 42.77 × 108 m3, 57.62 × 108 m3, 44.47 × 108 m3 and 48.54 × 108 m3, respectively, which showed a trend that at first increased and then decreased with time; also in the spatial dimension, there was a trend of being low in the southwest and slightly higher in the northeast. The total water yield for the different land use types was as follows: farmland > construction land > woodland > grassland > water and unused land. The total soil conservations were 11.34 × 108 t, 13.73 × 108 t, 12.37 × 108 t and 13.53 × 108 t, respectively. In terms of time, the soil conservation increased at first and then decreased, and in terms of spatial distribution was higher in the southwest at higher elevations, and lower in the northeast plain. The order for the amount of soil conservation in the different land use types was as follows: woodland > farmland > grassland > construction land > water > unused land. The carbon storage for each period were 1.262 × 108 t, 1.263 × 108 t, 1.252 × 108 t and 1.246 × 108 t, respectively, and the spatial distribution pattern did not change significantly. Specifically, the carbon storage of lush vegetation in the southwest was high, that of the plains in the middle and lower reaches was moderate, those for construction land and water were low and showed a trend at first increased and then decreased with time. The total carbon storage in the different land types was in the order: woodland > farmland > grassland > unused land > water. The food supplies were 0.74 × 107 t, 1.28 × 107 t, 1.88 × 107 t and 2.06 × 107 t, respectively. Over time, the food supply increased continuously and the rate of the contribution from farmland became larger. (3) The Spearman rank correlation coefficient showed that the trade-offs for different ecosystem services in the Yellow River Basin (Henan section) were dominant before 2000, and the synergies strengthened gradually after 2000, in which soil conservation—carbon storage showed strong synergy, water yield—soil conservation showed strong trade-off and was relatively stable and water yield—food supply, food supply—soil conservation and carbon storage—food supply changed from trade-off to synergy. (4) Ecosystem service functions have different relationships for the different land use types, and the region presents significant spatial heterogeneity. The ecosystem service functions in the middle reaches and lower reaches of the Yellow River basin (Henan section) were mainly trade-offs, while those in the regions with higher elevations in the middle reaches were mainly synergies. The trade-off between water yield and carbon storage on construction land was the strongest, and the synergy between carbon storage and food supply was the strongest for unused land and woodland.